1. Field
The present invention relates to data communications systems, and more particularly to methods for detecting the collision of messages transmitted along a multipoint data communication network.
2. Art Background
Multipoint data communications networks permit a plurality of stations, such as for example computers, terminals, printers, analog devices, etc., to communicate over one or more shared communication channels.
In many broad-band local area networks, each station communicates with other stations in the network using well known frequency division multiplexing techniques. Transmissions from all stations are coupled to a common transmit line frequently referred to as the "up-channel", to an electrical retransmission device known as a "head end". The head end retransmits the signals to a common receive line generally referred to as the "down channel". Each station is coupled to the respective common transmit and receive lines by means of a modem. Thus, the transmitter (the modulator) in each modem is coupled to the upchannel and the receiver (the demodulator) is coupled to the down-channel.
In some broad band network systems, the up-channel and the down-channel share a single coaxial cable. The two channels operate at two different carrier frequencies in order to permit the modems of each station to distinguish between transmissions and receptions. In such systems, the head end device translates the transmitted signals to the appropriate frequency for reception.
In other broad band systems, two separate communications lines are utilized, such that the head end device retransmits signals coupled to the up-channels onto the down-channel without the necessity for frequency translation.
Where long cable lengths are used, the delay time for a transmitted signal to travel along the transmit up-channel to the head end and be received by the transmitting station on the receive down-channel can become significant. For example, referring briefly to FIG. 1, assume that station Z begins to transmit a message at time t.sub.0. Before Z's signal can pass through the head end and be detected on the receive channel by transceiver Y, assume Y not sensing any cable use, begins to transmit its own message at time t.sub.1. A collision of signals results, and thus some collision detection mechanism is needed in the communication system in order to avoid chaos.
The present invention provides a collision detection method which incorporates the use of code schemes commonly used to verify that an error free message has been received. Accordingly, the present invention is easy to implement and is compatible with cable priority access methods such as carrier sensed multiple access (CSMA) (See for example, U.S. Pat. No. 4,234,952).